Hybrid vehicle and method of controlling an engine disconnect clutch for engine start-up

ABSTRACT

A vehicle includes an engine, a transmission, a clutch, and a controller. The clutch is configured to couple the engine and transmission during engine starts. The controller is programmed to, in response to an actual engine start time being greater than an upper threshold time for an engine start event, alter an engine start torque apply schedule for the clutch such that the actual engine start time is less than the upper threshold time for a next engine start event. The controller may be further programmed to, in response to the actual engine start time being less than a lower threshold time for the engine start event, alter the engine start torque apply schedule for the clutch such that the actual engine start time is greater than the lower threshold time for the next engine start event.

REFERENCE TO RELATED APPLICATION

This application is a division of U.S. application Ser. No. 14/802,335filed Jul. 17, 2015, the disclosure of which is hereby incorporated inits entirety by reference herein.

TECHNICAL FIELD

The present disclosure relates to hybrid vehicles and adjusting thetorque of an engine disconnect clutch during engine starting events.

BACKGROUND

Hybrid vehicles may include an engine disconnect clutch that isconfigured to disconnect an internal combustion engine from the vehiclepowertrain. The engine disconnect clutch may disconnect the engine fromthe powertrain when the vehicle is operating in an electric only mode inorder to improve fuel efficiency.

SUMMARY

A vehicle is provided. The vehicle includes an engine, a transmission, aclutch, and a controller. The clutch is configured to couple the engineand transmission during engine starts. The controller is programmed to,in response to an actual engine start time being less than a lowerthreshold time for an engine start event, alter an engine start torqueapply schedule for the clutch such that the actual engine start time isgreater than the lower threshold time for a next engine start event.

A method of operating a clutch configured to couple an engine to atransmission of a vehicle during engine starts is provided. The methodincludes, in response to an actual engine start time being less than alower threshold time for an engine start event, altering an engine starttorque apply schedule for the clutch such that the actual engine starttime is greater than the lower threshold time for a subsequent enginestart event.

A vehicle is provided. The vehicle includes an engine, a transmission, aclutch, and a controller. The clutch is configured to couple the engineto the transmission during engine starts. The controller is programmedto, in response to an actual engine start time falling outside a targettime range, alter an engine start torque apply schedule for the clutchsuch that the actual engine start time falls within the target timerange for a subsequent engine start event.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an example powertrain of a hybridelectric vehicle;

FIG. 2 is a graphical representation of a bump start of an engine in ahybrid electric vehicle;

FIG. 3 is graphical representation of a ramp start of an engine in ahybrid electric vehicle; and

FIG. 4 is a flow chart of an algorithm for adjusting the torque of adisconnect clutch during an engine starting event.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments may take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the presentinvention. As those of ordinary skill in the art will understand,various features illustrated and described with reference to any one ofthe figures may be combined with features illustrated in one or moreother figures to produce embodiments that are not explicitly illustratedor described. The combinations of features illustrated providerepresentative embodiments for typical applications. Variouscombinations and modifications of the features consistent with theteachings of this disclosure, however, could be desired for particularapplications or implementations.

Referring to FIG. 1, a schematic diagram of a hybrid electric vehicle(HEV) 10 is illustrated according to an embodiment of the presentdisclosure. FIG. 1 illustrates representative relationships among thecomponents. Physical placement and orientation of the components withinthe vehicle may vary. The HEV 10 includes a powertrain 12. Thepowertrain 12 includes an engine 14 that drives a transmission 16, whichmay be referred to as a modular hybrid transmission (MHT). As will bedescribed in further detail below, transmission 16 includes an electricmachine such as an electric motor/generator (M/G) 18, an associatedtraction battery 20, a torque converter 22, and a multiple step-ratioautomatic transmission, or gearbox 24.

The engine 14 and the M/G 18 are both drive sources for the HEV 10. Theengine 14 generally represents a power source that may include aninternal combustion engine such as a gasoline, diesel, or natural gaspowered engine, or a fuel cell. The engine 14 generates an engine powerand corresponding engine torque that is supplied to the M/G 18 when adisconnect clutch 26 between the engine 14 and the M/G 18 is at leastpartially engaged. The M/G 18 may be implemented by any one of aplurality of types of electric-machines. For example, M/G 18 may be apermanent magnet synchronous motor. Power electronics condition directcurrent (DC) power provided by the battery 20 to the requirements of theM/G 18, as will be described below. For example, power electronics mayprovide three phase alternating current (AC) to the M/G 18.

When the disconnect clutch 26 is at least partially engaged, power flowfrom the engine 14 to the M/G 18 or from the M/G 18 to the engine 14 ispossible. For example, the disconnect clutch 26 may be engaged and M/G18 may operate as a generator to convert rotational energy provided by acrankshaft 28 and M/G shaft 30 into electrical energy to be stored inthe battery 20. A flywheel 29, which may be dual mass flywheel, may bedisposed on the crankshaft 28 between the engine 14 and the disconnectclutch 26. The disconnect clutch 26 can also be disengaged to isolatethe engine 14 from the remainder of the powertrain 12 such that the M/G18 can act as the sole drive source for the HEV 10. Shaft 30 extendsthrough the M/G 18. The M/G 18 is continuously drivably connected to theshaft 30, whereas the engine 14 is drivably connected to the shaft 30only when the disconnect clutch 26 is at least partially engaged.

The M/G 18 is connected to the torque converter 22 via shaft 30. Thetorque converter 22 is therefore connected to the engine 14 when thedisconnect clutch 26 is at least partially engaged. The torque converter22 includes an impeller fixed to M/G shaft 30 and a turbine fixed to atransmission input shaft 32. The torque converter 22 thus provides ahydraulic coupling between shaft 30 and transmission input shaft 32. Thetorque converter 22 transmits power from the impeller to the turbinewhen the impeller rotates faster than the turbine. The magnitude of theturbine torque and impeller torque generally depend upon the relativespeeds. When the ratio of impeller speed to turbine speed issufficiently high, the turbine torque is a multiple of the impellertorque. A torque converter bypass clutch (also known as a torqueconverter lock-up clutch) 34 may also be provided that, when engaged,frictionally or mechanically couples the impeller and the turbine of thetorque converter 22, permitting more efficient power transfer. Thetorque converter bypass clutch 34 may be operated as a launch clutch toprovide smooth vehicle launch. Alternatively, or in combination, alaunch clutch similar to disconnect clutch 26 may be provided betweenthe M/G 18 and gearbox 24 for applications that do not include a torqueconverter 22 or a torque converter bypass clutch 34. In someapplications, disconnect clutch 26 is generally referred to as anupstream clutch and launch clutch 34 (which may be a torque converterbypass clutch) is generally referred to as a downstream clutch.

The gearbox 24 may include gear sets (not shown) that are selectivelyplaced in different gear ratios by selective engagement of frictionelements such as clutches and brakes (not shown) to establish thedesired multiple discrete or step drive ratios. The friction elementsare controllable through a shift schedule that connects and disconnectscertain elements of the gear sets to control the ratio between atransmission output shaft 36 and the transmission input shaft 32. Thegearbox 24 is automatically shifted from one ratio to another based onvarious vehicle and ambient operating conditions by an associatedcontroller, such as a powertrain control unit (PCU). The gearbox 24 thenprovides powertrain output torque to output shaft 36.

It should be understood that the hydraulically controlled gearbox 24used with a torque converter 22 is but one example of a gearbox ortransmission arrangement; any multiple ratio gearbox that accepts inputtorque(s) from an engine and/or a motor and then provides torque to anoutput shaft at the different ratios is acceptable for use withembodiments of the present disclosure. For example, gearbox 24 may beimplemented by an automated mechanical (or manual) transmission (AMT)that includes one or more servo motors to translate/rotate shift forksalong a shift rail to select a desired gear ratio. As generallyunderstood by those of ordinary skill in the art, an AMT may be used inapplications with higher torque requirements, for example.

As shown in the representative embodiment of FIG. 1, the output shaft 36is connected to a differential 40. The differential 40 drives a pair ofwheels 42 via respective axles 44 connected to the differential 40. Thedifferential transmits approximately equal torque to each wheel 42 whilepermitting slight speed differences such as when the vehicle turns acorner. Different types of differentials or similar devices may be usedto distribute torque from the powertrain to one or more wheels. In someapplications, torque distribution may vary depending on the particularoperating mode or condition, for example.

The powertrain 12 further includes an associated controller 50 such as apowertrain control unit (PCU). While illustrated as one controller, thecontroller 50 may be part of a larger control system and may becontrolled by various other controllers throughout the vehicle 10, suchas a vehicle system controller (VSC). It should therefore be understoodthat the powertrain control unit 50 and one or more other controllerscan collectively be referred to as a “controller” that controls variousactuators in response to signals from various sensors to controlfunctions such as starting/stopping engine 14, operating M/G 18 toprovide wheel torque or charge battery 20, select or scheduletransmission shifts, etc. Controller 50 may include a microprocessor orcentral processing unit (CPU) in communication with various types ofcomputer readable storage devices or media. Computer readable storagedevices or media may include volatile and nonvolatile storage inread-only memory (ROM), random-access memory (RAM), and keep-alivememory (KAM), for example. KAM is a persistent or non-volatile memorythat may be used to store various operating variables while the CPU ispowered down. Computer-readable storage devices or media may beimplemented using any of a number of known memory devices such as PROMs(programmable read-only memory), EPROMs (electrically PROM), EEPROMs(electrically erasable PROM), flash memory, or any other electric,magnetic, optical, or combination memory devices capable of storingdata, some of which represent executable instructions, used by thecontroller in controlling the engine or vehicle.

The controller communicates with various engine/vehicle sensors andactuators via an input/output (I/O) interface that may be implemented asa single integrated interface that provides various raw data or signalconditioning, processing, and/or conversion, short-circuit protection,and the like. Alternatively, one or more dedicated hardware or firmwarechips may be used to condition and process particular signals beforebeing supplied to the CPU. As generally illustrated in therepresentative embodiment of FIG. 1, controller 50 may communicatesignals to and/or from engine 14, disconnect clutch 26, M/G 18, battery20, launch clutch 34, transmission gearbox 24, and power electronics 56.Although not explicitly illustrated, those of ordinary skill in the artwill recognize various functions or components that may be controlled bycontroller 50 within each of the subsystems identified above.Representative examples of parameters, systems, and/or components thatmay be directly or indirectly actuated using control logic executed bythe controller include fuel injection timing, rate, and duration,throttle valve position, spark plug ignition timing (for spark-ignitionengines), intake/exhaust valve timing and duration, front-end accessorydrive (FEAD) components such as an alternator, air conditioningcompressor, battery charging or discharging (including determining themaximum charge and discharge power limits), regenerative braking, M/Goperation, clutch pressures for disconnect clutch 26, launch clutch 34,and transmission gearbox 24, and the like. Sensors communicating inputthrough the I/O interface may be used to indicate turbocharger boostpressure, crankshaft position (PIP), engine rotational speed (RPM),wheel speeds (WS1, WS2), vehicle speed (VSS), coolant temperature (ECT),intake manifold pressure (MAP), accelerator pedal position (PPS),ignition switch position (IGN), throttle valve position (TP), airtemperature (TMP), exhaust gas oxygen (EGO) or other exhaust gascomponent concentration or presence, intake air flow (MAF), transmissiongear, ratio, or mode, transmission oil temperature (TOT), transmissionturbine speed (TS), torque converter bypass clutch 34 status (TCC),deceleration or shift mode (MDE), battery temperature, voltage, current,or state of charge (SOC) for example.

Control logic or functions performed by controller 50 may be representedby flow charts or similar diagrams in one or more figures. These figuresprovide representative control strategies and/or logic that may beimplemented using one or more processing strategies such asevent-driven, interrupt-driven, multi-tasking, multi-threading, and thelike. As such, various steps or functions illustrated may be performedin the sequence illustrated, in parallel, or in some cases omitted.Although not always explicitly illustrated, one of ordinary skill in theart will recognize that one or more of the illustrated steps orfunctions may be repeatedly performed depending upon the particularprocessing strategy being used. Similarly, the order of processing isnot necessarily required to achieve the features and advantagesdescribed herein, but is provided for ease of illustration anddescription. The control logic may be implemented primarily in softwareexecuted by a microprocessor-based vehicle, engine, and/or powertraincontroller, such as controller 50. Of course, the control logic may beimplemented in software, hardware, or a combination of software andhardware in one or more controllers depending upon the particularapplication. When implemented in software, the control logic may beprovided in one or more computer-readable storage devices or mediahaving stored data representing code or instructions executed by acomputer to control the vehicle or its subsystems. The computer-readablestorage devices or media may include one or more of a number of knownphysical devices which utilize electric, magnetic, and/or opticalstorage to keep executable instructions and associated calibrationinformation, operating variables, and the like.

An accelerator pedal 52 is used by the driver of the vehicle to providea demanded torque, power, or drive command to propel the vehicle. Ingeneral, depressing and releasing the pedal 52 generates an acceleratorpedal position signal that may be interpreted by the controller 50 as ademand for increased power or decreased power, respectively. Based atleast upon input from the pedal, the controller 50 commands torque fromthe engine 14 and/or the M/G 18. The controller 50 also controls thetiming of gear shifts within the gearbox 24, as well as engagement ordisengagement of the disconnect clutch 26 and the torque converterbypass clutch 34. Like the disconnect clutch 26, the torque converterbypass clutch 34 can be modulated across a range between the engaged anddisengaged positions. This produces a variable slip in the torqueconverter 22 in addition to the variable slip produced by thehydrodynamic coupling between the impeller and the turbine.Alternatively, the torque converter bypass clutch 34 may be operated aslocked or open without using a modulated operating mode depending on theparticular application.

To drive the vehicle with the engine 14, the disconnect clutch 26 is atleast partially engaged to transfer at least a portion of the enginetorque through the disconnect clutch 26 to the M/G 18, and then from theM/G 18 through the torque converter 22 and gearbox 24. The M/G 18 mayassist the engine 14 by providing additional power to turn the shaft 30.This operation mode may be referred to as a “hybrid mode” or an“electric assist mode.” If the M/G 18 is not assisting the engine 14 byproviding additional power to turn the shaft 30, this operation mode maybe referred to as an “engine only” mode.

To drive the vehicle with the M/G 18 as the sole power source, the powerflow remains the same except the disconnect clutch 26 isolates theengine 14 from the remainder of the powertrain 12. Combustion in theengine 14 may be disabled or otherwise OFF during this time to conservefuel. The traction battery 20 transmits stored electrical energy throughwiring 54 to power electronics 56 that may include an inverter, forexample. The power electronics 56 convert DC voltage from the battery 20into AC voltage to be used by the M/G 18. The controller 50 commands thepower electronics 56 to convert voltage from the battery 20 to an ACvoltage provided to the M/G 18 to provide positive or negative torque tothe shaft 30. This operation mode may be referred to as an “electriconly” operation mode.

In any mode of operation, the M/G 18 may act as a motor and provide adriving force for the powertrain 12. Alternatively, the M/G 18 may actas a generator and convert kinetic energy from the powertrain 12 intoelectric energy to be stored in the battery 20. The M/G 18 may act as agenerator while the engine 14 is providing propulsion power for thevehicle 10, for example. The M/G 18 may additionally act as a generatorduring times of regenerative braking in which rotational energy fromspinning wheels 42 is transferred back through the gearbox 24 and isconverted into electrical energy for storage in the battery 20.

The M/G 18 may be used to crank and start the engine 14 when the HEV 10is transitioning to the “hybrid mode” or “engine only” mode from the“electric only” mode or from condition where neither the M/G 18 orengine 14 are transferring power through the powertrain 12. Power istransferred from the M/G 18 to engine 14 in order to start the engine 14by at least partially engaging the disconnect clutch 26 to transfer atleast a portion of the M/G 18 torque through the disconnect clutch 26 tothe engine 14.

The disconnect clutch 26 may be any type of clutch known by one who isskilled in the art. For example, the disconnect clutch 26 may be ahydraulic clutch or dry clutch. If the disconnect clutch 26 is ahydraulic clutch, hydraulic fluid is utilized to engage or disengageopposing sides of the clutch. It should be noted that the hydraulicfluid may be the transmission oil that is also used in the torqueconverter 22 and gearbox 24 of the transmission 16. If the disconnectclutch 26 is a hydraulic clutch, the torque of the disconnect clutchwhile engaging, T_(DCC), may be based on Equation (1):

T _(DCC)=mu(P _(apply) −P _(stroke))A _(clutch)  (1)

mu is the clutch coefficient of friction, which is a function ofhydraulic fluid temperature and the temperature of the clutchcomponents.P_(apply) is the clutch apply pressure generated by advancing the pistonto engage the opposing sides of the clutch.P_(stroke) is a return pressure that biases the hydraulic clutch into anopen position. The return pressure may be generated by a spring,hydraulic pressure, pneumatic pressure, electrical mechanical devices,or other appropriate means. P_(stroke) is the equal to P_(apply) whenthe clutch just begins to transmit torque. It should be noted thatT_(DCC) equals zero until P_(stroke) obtains a value that exceedsP_(apply).A_(clutch) is the effective area of engagement between the opposingsides the hydraulic clutch.

Alternatively, if the disconnect clutch 26 is a dry clutch, the clutchapply pressure may be generated by means other than hydraulic pressure.For example, the clutch pressure may be generated by a pneumatic device(e.g., pneumatic cylinder) or an electrical mechanical device (e.g.,electric motor, electric solenoid, etc.). The torque of a dry disconnectclutch while engaging, T_(DDC), may be based on an equation similar toEquation 1, however, the clutch apply pressure, P_(apply), would be afunction of the device (e.g., pneumatic or electromechanical device)utilized to generate the clutch apply pressure, and the coefficient offriction, mu, would be a function of the clutch components alone (whichmay take into account the temperature of the clutch components).

The time it takes for the engine 14 to start-up (i.e., the enginestart-up time), during an engine starting event where power istransferred from the M/G 18 to the engine 14 via the disconnect clutch26, may be divided into two distinct time periods. The first time periodmay correspond to a cranking time of the engine 14. The cranking timemay correspond to the time it takes to crank the engine 14 from astopped or shutdown position to a first firing top dead center positionwere fuel and spark are applied and combustion of the engine 14commences. During the cranking time, the crankshaft 28 is rotatedinitiating movement of the pistons and other internal components of theengine 14 until the first firing top dead center position is obtained.

The cranking time of the engine 14 may be may be based on Equations (2)and (3):

$\begin{matrix}{{\Delta\theta} = {{\omega_{0}t} + {\frac{1}{2}\alpha_{ave}t^{2}}}} & (2) \\{{I_{crank}\alpha_{ave}} = {T_{DCC\_ ave} - T_{comp\_ ave}}} & (3)\end{matrix}$

Δθ is angular displacement of the crankshaft 28, which may correspond tothe angular displacement of the crankshaft 28 during the cranking timeof the engine 14.ω₀ is an initial angular velocity of the crankshaft 28, which maycorrespond to a value of zero during the cranking time of the engine 14.α_(ave) is the average angular acceleration of the crankshaft 28, whichmay correspond to an average acceleration of the crankshaft 28 duringthe cranking time of the engine 14.t is the cranking time of the engine 14, which may correspond to thetime it takes to rotate the crankshaft 28 from the stopped or shutdownposition to the first firing top dead center position.I_(crank) is the moment of inertia of the crankshaft 28 and anycorresponding pistons that are connected to the crankshaft.T_(DCC) _(_) _(ave) is the average torque of the disconnect clutch 26,which may correspond to the average torque of disconnect clutch 26during the cranking time of the engine 14.T_(comp) _(_) _(ave) is the average cylinder pressure compression torquethat results from the pistons of the engine 14 moving within thecylinders of the engine 14, which may correspond to the average cylinderpressure compression torque during the cranking time of the engine.

Equations (2) and (3) may also adapted to take into account the internalfriction forces of the engine 14 components (e.g., pistons, crankshaft,etc.).

The second time period, immediately following the cranking time of theengine 14, may correspond to a run-up time of the engine 14. During therun-up time, the combustion of the engine 14 is controlled until theengine 14 reaches the output speed of the disconnect clutch 26 (theoutput speed of the disconnect clutch 26 may correspond to the speed ofthe M/G 18). The combustion of the engine 14 may be controlled duringthe run-up time to obtain an engine speed using a feedback of the outputspeed of the disconnect clutch 26.

It should be understood that the schematic illustrated in FIG. 1 ismerely exemplary and is not intended to be limited. Other configurationsare contemplated that utilize selective engagement of both an engine anda motor to transmit through the transmission. For example, the M/G 18may be offset from the crankshaft 28, an additional motor may beprovided to start the engine 14, and/or the M/G 18 may be providedbetween the torque converter 22 and the gearbox 24. Other configurationsare contemplated without deviating from the scope of the presentdisclosure.

Although the embodiment illustrated in FIG. 1 depicts a parallel hybridvehicle structure, the disclosure should be construed to include otherhybrid vehicle configurations including series hybrid vehicles,series-parallel hybrid vehicles, power-split hybrid vehicles, plug-inhybrid electric vehicles (PHEVs), electric-fuel cell hybrid vehicles,micro hybrid vehicles (vehicles with engine start-stop systems), or anyother hybrid vehicle configuration known in the art.

Referring to FIG. 2, a graphical representation of a scheduled bumpstart of the engine 14 is illustrated. During a bump start, the engine14 is started by transferring energy from the transmission 16 or M/G 18to the engine 14 by at least partially engaging the disconnect clutch26. A bump start may be implemented and executed by the controller 50.Line 58 represents a scheduled torque applied by the disconnect clutch26 plotted against time, line 60 represents a scheduled torque of theengine 14 plotted against time, line 62 represents a scheduled speed ofthe engine (which may correspond to an angular velocity of thecrankshaft 28, ω_(crank)) plotted against time, and line 64 represents ascheduled speed of the M/G 18 (which may correspond to the output speedof the disconnect clutch 26 and angular velocity of M/G shaft 30, ω_(mg)_(_) _(shaft)) plotted against time.

The scheduled start-up time of the engine 14 spans the time periodbetween t₀ and t₂, and is represented by line 66. The scheduled crankingtime of the engine 14 spans the time period between t₀ and t₁, and isrepresented by line 68. The scheduled run-up time of the engine 14 spansthe time period between t₁ and t₂, and is represented by line 70. Thescheduled start-up time 66, scheduled cranking time 68, and scheduledrun-up time 70 may be constant values or may be a range of values thatspan upper and lower thresholds. The scheduled start-up time 66 maycorrespond to a target start-up time. The target start-up time may be aconstant value or a range of values between the upper and lowerthresholds of the scheduled start-up time 66. The scheduled crankingtime 68 may correspond to a target cranking time. The target crankingtime may be a constant value or a range of values between the upper andlower thresholds of the scheduled cranking time 68. The scheduled run-uptime 70 may correspond to a target run-up time. The target run-up timemay be a constant value or a range of values between the upper and lowerthresholds of the scheduled run-up time 70. During the scheduledcranking time 68, the disconnect clutch 26 is closed and the torque ofdisconnect clutch 26 is quickly ramped up. The torque of the disconnectclutch 26 may be ramped up to a constant value or to a ramped value thatcontinues to increase in torque until the scheduled cranking time 68ends. The torque from the disconnect clutch 26 increases the speed ofthe engine 14 from a stopped or shutdown position at t₀ to a firstfiring top dead center position at t₁, during the scheduled crankingtime 68. Time t₀ may correspond to the initial contact between the inputand the output of the disconnect clutch 26 during the closing of thedisconnect clutch 26.

Once the engine has reached the first firing top dead center position,at time t₁, the scheduled run-up time 70 of the engine 14 commences,where fuel and spark are applied, and combustion of the engine 14commences. During the scheduled run-up time 70, the combustion of theengine 14 is controlled to increase the torque and speed of the engine14 until the engine 14 obtains the speed of the M/G 18 and/or the outputspeed of the disconnect clutch 26 at time t₂. Also, during the scheduledrun-up time 70 of a bump start, the torque of the disconnect clutch 26is decreased until the engine 14 obtains the speed of the M/G 18 and theoutput speed of the disconnect clutch 26 at time t₂. Once the engine hasobtained the speed of the M/G 18 and the output speed of the disconnectclutch 26 at time t₂, the torque of the disconnect clutch 26 is rampedup in order to lock the disconnect clutch 26 which occurs at time t₃.

If an actual start-up time or an actual cranking time differs from thescheduled start-up time 66 or the scheduled cranking time 68,respectively, during a bump start, the torque of the disconnect clutch26 may be adjusted up or down, by the controller 50, during thescheduled cranking time 68 of a subsequent starting event of the engine14, as indicated by line 72. The scheduled torque applied by thedisconnect clutch 26 may be adjusted (during the scheduled start-up time66 of the subsequent starting event of the engine 14) such that theactual start-up time is greater than a lower threshold of the scheduledstart-up time 66 and/or less than an upper threshold of the scheduledstart-up time 66. Alternatively, the scheduled torque applied by thedisconnect clutch 26 may be adjusted (during the scheduled cranking time68 of the subsequent starting event of the engine 14) such that theactual cranking time is greater than a lower threshold of the scheduledcranking time 68 and/or less than an upper threshold of the scheduledcranking time 68.

Referring to FIG. 3, a graphical representation of a scheduled rampstart of the engine 14 is illustrated. During a ramp start, the engine14 is started by transferring energy from the transmission 16 or M/G 18to the engine 14 by at least partially engaging the disconnect clutch26. A ramp start may be implemented and executed by the controller 50.Line 58′ represents a scheduled torque applied by the disconnect clutch26 plotted against time, line 60′ represents a scheduled torque of theengine plotted against time, line 62′ represents a scheduled speed ofthe engine (which may correspond to an angular velocity of thecrankshaft 28, ω_(crank)) plotted against time, and line 64′ representsa scheduled speed of the M/G 18 (which may correspond to the outputspeed of the disconnect clutch 26 and angular velocity of M/G shaft 30,ω_(mg) _(_) _(shaft)) plotted against time.

The scheduled start-up time of the engine 14 spans the time periodbetween t₀ and t₂, and is represented by line 66′. The scheduledcranking time of the engine 14 spans the time period between t₀ and t₁,and is represented by line 68′. The scheduled run-up time of the engine14 spans the time period between t₁ and t₂, and is represented by line70′. The scheduled start-up time 66′, scheduled cranking time 68′, andscheduled run-up time 70′ may be constant values or may be a range ofvalues that span upper and lower thresholds. The scheduled start-up time66′ may correspond to a target start-up time. The target start-up timemay be a constant value or a range of values between the upper and lowerthresholds of the scheduled start-up time 66′. The scheduled crankingtime 68′ may correspond to a target cranking time. The target crankingtime may be a constant value or a range of values between the upper andlower thresholds of the scheduled cranking time 68′. The scheduledrun-up time 70′ may correspond to a target run-up time. The targetrun-up time may be a constant value or a range of values between theupper and lower thresholds of the scheduled run-up time 70′. During thescheduled cranking time 68′, the disconnect clutch 26 is closed and thetorque of disconnect clutch 26 is quickly ramped up. The torque of thedisconnect clutch 26 may be ramped up to a constant value or to a rampedvalue that continues to increase in torque until the scheduled crankingtime 68′ ends. The torque from the disconnect clutch 26 increases thespeed of the engine 14 from a stopped or shutdown position at t₀ to afirst firing top dead center position at t₁, during the scheduledcranking time 68′. Time t₀ may correspond to the initial contact betweenthe input and the output of the disconnect clutch 26 during the closingof the disconnect clutch 26.

Once the engine has reached the first firing top dead center position,at time t₁, the scheduled run-up time 70′ of the engine 14 commences,where fuel and spark are applied, and combustion of the engine 14commences. During the scheduled run-up time 70′, the combustion of theengine 14 is controlled to increase the torque and speed of the engine14 until the engine 14 obtains the speed of the M/G 18 and/or the outputspeed of the disconnect clutch 26 at time t₂. Also, during the scheduledrun-up time 70′ of a ramp start, the torque of the disconnect clutch 26is either held at the cranking value, raised to an intermediate constantvalue, or raised to ramped value that continues to increase in torqueuntil the engine 14 obtains the speed of the M/G 18 and the output speedof the disconnect clutch 26 at time t₂. Once the engine has obtained thespeed of the M/G 18 and the output speed of the disconnect clutch 26 attime t₂, the torque of the disconnect clutch 26 is ramped up in order tolock the disconnect clutch 26 which occurs at time t₃.

If an actual start-up time, an actual cranking time, or actual run-uptime differs from the scheduled start-up time 66′, scheduled crankingtime 68′, or scheduled run-up time 70′, respectively, during a rampstart, the torque of the disconnect clutch 26 may be adjusted up ordown, by the controller 50, during the scheduled cranking time 68′ of asubsequent starting event of the engine 14, as indicated by line 72′,and/or during the scheduled run-up time 70′ of the subsequent startingevent of the engine 14, as indicated by line 74′. The scheduled torqueapplied by the disconnect clutch 26 may be adjusted (during thescheduled cranking time 68′ and/or the scheduled run-up time 70′ of thesubsequent starting event of the engine 14) such that the actualstart-up time is greater than a lower threshold of the scheduledstart-up time 66′ and/or less than an upper threshold of the scheduledstart-up time 66′. Alternatively, the scheduled torque applied by thedisconnect clutch 26 may be adjusted (during the scheduled cranking time68′ of the subsequent starting event of the engine 14) such that theactual cranking time is greater than a lower threshold of the scheduledcranking time 68′ and/or less than an upper threshold of the scheduledcranking time 68′. In another alternative, the scheduled torque appliedby the disconnect clutch 26 may be adjusted (during the scheduled run-uptime 70′ of the subsequent starting event of the engine 14) such thatthe actual run-up time is greater than a lower threshold of thescheduled run-up time 70′ and/or less than an upper threshold of thescheduled run-up time 70′.

The target start-up time, target cranking time, and target run-up time,during either a bump start or a ramp start, may be functions of theshutdown position of the engine 14 relative to the first firing top deadcenter position, barometric pressure, intake manifold pressure, enginecoolant temperature, engine oil temperature, air charge temperature,input speed of the transmission (which may correspond to the outputspeed of the disconnect clutch 26 and the speed of the M/G 18), and thehydraulic fluid temperature (if the disconnect clutch 26 is a hydraulicclutch).

The target start-up time, target cranking time, and/or target run-uptime may increase as the angular displacement required or the amount ofcylinder pressure compression that must be overcome in order to crankthe engine 14 to the first firing top dead center position increases.The angular displacement required and the amount of cylinder pressurecompression that must be overcome in order to crank the engine 14 to thefirst firing top dead center position are both functions of the shutdownposition of the engine 14 relative to the first firing top dead centerposition and increase as the shutdown position of the engine 14 relativeto the first firing top dead center position increases.

Additionally, the target start-up time, target cranking time, and/ortarget run-up time may increase as the cylinder pressure compressiontorque increases while cranking the engine 14. As the barometricpressure and/or intake manifold pressure increases, an increase in thecylinder pressure compression torque may result, in turn increasing thetarget start-up time, target cranking time, and/or target run-up.

The engine 14 may operate more efficiently once the temperatures of theengine coolant and engine oil are above certain temperature values. Whenan internal combustion engine is operating at a temperature below thetemperature threshold, internal engine friction losses (includingfriction losses that occur at the piston to cylinder ring interfaces,various bearings, and valve train components) may increase. This is afunction of engine oil and metal temperatures being below a threshold,both of which are influenced by engine coolant temperature. Also, fueltends to form a film on the internal surfaces of the air intakecomponents of the engine, which disrupts the air/fuel control systemwhen the engine 14 is operating below the threshold temperature. Asfriction losses of the engine 14 increase, due to the engine coolanttemperature and engine oil temperature being below a threshold, anincrease in the target start-up time, target cranking time, and/ortarget run-up time may be required to overcome the increased friction.Additionally, an increase in the target start-up time, target crankingtime, and/or target run-up time may be required to overcome thedisrupted air/fuel mixture that occurs while the engine 14 is operatingbelow a threshold.

The air/fuel mixture may also be affected by the air charge temperature,which refers to the air density. As the air density decreases, theamount of fuel that may be injected into the cylinders of the engine 14will also decrease. A decrease in the amount fuel may lead to a decreasein the power the engine 14 is capable of producing, which in turn maylead to an increase in the target start-up time, target cranking time,and/or target run-up time.

The friction between the moving parts of the disconnect clutch 26 maychange as the temperature of the components of the disconnect clutch 26and/or the temperature of the hydraulic fluid (if the disconnect clutch26 is a hydraulic clutch) changes. This may affect the amount of torquethat may be transferred between the M/G 18 and the engine 14, and theamount of torque that may be transferred between the opposing sides ofthe disconnect clutch 26. Therefore, the target start-up time, targetcranking time, and/or target run-up time may need to be adjusteddepending on how the temperature of the components of the disconnectclutch 26 and/or the temperature of the hydraulic fluid effects theamount of torque that may be transferred between the M/G 18 and engine14.

The target start-up time, target cranking time, and/or target run-up mayalso need to be adjusted base on input speed of the transmission (whichmay correspond to the output speed of the disconnect clutch 26 and thespeed of the M/G 18). Since starting the engine 14 requires bringing theengine speed up to the input speed of the transmission, an increase inthe input speed of the transmission will result in an increased targetstart-up time, target cranking time, and/or target run-up time.

Referring to FIG. 4, a method 100 of adjusting the torque of thedisconnect clutch 26 during an engine starting event is illustrated. Themethod may be applicable to both ramp starts and bumps starts of theengine 14, discussed above. The method 100 may be implemented andexecuted by the controller 50. The method 100 is initiated at the startblock 102. The method 100 may be initiated by placing a vehicle ignitioninto an “on” position, pressing a “start/run” button, placing thetransmission of the HEV 10 into a specific gear selection, or by anyother appropriate condition of the HEV 10.

After the method 100 is initiated at step 102, the method moves on tostep 104 where it is determined if the engine 14 is being started byclosing the disconnect clutch 26 between the engine 14 and transmission16 or the M/G 18. If the engine 14 is not being started by closing thedisconnect clutch 26, the method 100 ends at step 106. If the engine 14is being started by closing the disconnect clutch 26, the method 100moves on to step 108.

At step 108, the actual engine start-up time is determined. The actualengine start-up time determined at step 108 may refer the actual totalstart-up time (the actual time it takes the engine 14 to reach theoutput speed of the disconnect clutch 26 and M/G 18 from a stopped orshutdown position), the actual cranking time (the actual time it takesthe engine 14 to reach the first firing top dead center position from astopped or shutdown position), or the actual run-up time (the actualtime it takes the engine 14 to reach the output speed of the disconnectclutch 26 and M/G 18 from the first firing top dead center position).The actual engine start-up time may refer to the start-up time of apreviously or recently recorded engine staring event. Alternatively, theactual engine start-up time may refer to the average of the actualstart-up times of a plurality of previously executed engine startingevents.

Once the actual engine start-up time is determined at step 108, themethod moves on to step 110 where the actual engine start-up time islimited to a range between a lower cutoff limit and an upper cutofflimit by a low-pass filter.

After the actual engine start-up time has been filtered at step 110, themethod 100 moves on to step 112 where it is determined if there is adifference between the actual engine start-up time and a target enginestart-up time. The target engine start-up time may also be determined atstep 112 and may refer to the total target start-up time (the targettime for the engine 14 to reach the output speed of the disconnectclutch 26 and M/G 18 from a stopped or shutdown position), the targetcranking time (the target time for the engine 14 to reach the firstfiring top dead center position from a stopped or shutdown position), orthe target run-up time (the target time for the engine 14 to reach theoutput speed of the disconnect clutch 26 and M/G 18 from the firstfiring top dead center position). The target engine start-up time mayhave a specific acceptable value or may have a range of acceptablevalues. If there is no a difference (or a large enough difference in thecase where a range of values is acceptable) between the actual enginestart-up time in the target engine start-up time, the method 100 ends atstep 106. If there is a difference (or a large enough difference in thecase where a range of values is acceptable) between the actual enginestart-up time and the target engine start-up time, the method 100 moveson to step 114.

At step 114, the torque of the disconnect clutch 26 is adjusted during asubsequent engine starting event (the torque adjusted at step 114 may bea scheduled torque applied by the disconnect clutch 26 during thesubsequent engine starting event, which may also comprise adjusting anengine start torque apply schedule for the disconnect clutch 26 for thesubsequent engine starting event). The engine start torque applyschedule for the disconnect clutch 26 may be adjusted such that theactual engine start time is less than the upper threshold or greaterthan a lower threshold during the subsequent engine starting event. Thetorque of disconnect clutch 26 may be adjusted during total start-uptime (the time it takes the engine 14 to reach the output speed of thedisconnect clutch 26 and M/G 18 from a stopped or shutdown position),the cranking time (the time it takes the engine 14 to reach the firstfiring top dead center position from a stopped or shutdown position), orthe run-up time (the time it takes the engine 14 to reach the outputspeed of the disconnect clutch 26 and M/G 18 from the first firing topdead center position) of the subsequent engine starting event. If thedisconnect clutch 26 is a hydraulic clutch, the torque of disconnectclutch 26 may be adjusted by adjusting the applied pressure, P_(apply),of the disconnect clutch 26. The torque (or applied pressure) of thedisconnect clutch 26 may be adjusted by an additive term, a multiplierterm, a look-up table, or may be adaptively learned. The torque (orapplied pressure) adjustment of the disconnect clutch 26 may be based ona ratio of the actual engine start-up time to the target engine start-uptime or may be based on a difference between the actual engine start-uptime and the target engine start-up time.

An example of adjusting the torque of the disconnect clutch during asubsequent engine starting event, may be based on Equation (4):

$\begin{matrix}{T_{sub} = {T_{actual}\left( {K_{adapt}*\frac{t_{actaul}}{t_{target}}} \right)}} & (4)\end{matrix}$

T_(su)b is the adjusted torque value during a subsequent engine startingevent.

T_(actual) is the actual torque of the disconnect clutch 26 during thecurrent or previous engine starting event.

K_(adapt) is an adaptive constant used as a multiplier term to adjustedtorque of disconnect clutch during a subsequent engine starting event.

t_(actual) is the actual start-up time of the engine 14 during thecurrent or previous engine starting event, which may correspond to theactual total start-up time, the actual cranking time, or the actualrun-up time.

t_(target) is the target start-up time of the engine 14 during thecurrent or previous engine starting event, which may correspond to thetotal target start-up time, the target cranking time, or the targetrun-up time.

It should be understood that the method 100 described in FIG. 4 ismerely descriptive and the disclosure should not be construed as limitedto the particular description in FIG. 4. Some of the steps in FIG. 4 maybe omitted and/or the chronological order of the particular steps may berearranged.

Additionally, the method 100 described in FIG. 4 may apply differentlywhen the engine starting event is a first start of a drive cycle. Whenan engine has not been running for a period of time, the oil lubricatingthe moving parts in the engine may settle leading to an increase infriction between the moving parts. The settled oil will be reapplied tothe moving parts once the engine is started, however, there may be anincreased start-up time during the first start of a drive cycle due tothe increase in friction. Therefore, the target start-up time may beadjusted when the method is applied to a first start of a drive cycle tocompensate for the increased friction. For example, the actual enginestart-up time may be an actual first engine start-up time of the drivecycle, the target engine start-up time may be a target first enginestart-up time of the first start of the drive cycle, and the torque ofthe disconnect clutch may be adjusted during a subsequent first enginestart of a subsequent drive cycle based on the actual first enginestart-up time of the drive cycle and the target first engine start-uptime of the first start of the drive cycle.

The words used in the specification are words of description rather thanlimitation, and it is understood that various changes may be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments may becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics may becompromised to achieve desired overall system attributes, which dependon the specific application and implementation. These attributes mayinclude, but are not limited to cost, strength, durability, life cyclecost, marketability, appearance, packaging, size, serviceability,weight, manufacturability, ease of assembly, etc. As such, embodimentsdescribed as less desirable than other embodiments or prior artimplementations with respect to one or more characteristics are notoutside the scope of the disclosure and may be desirable for particularapplications.

What is claimed is:
 1. A vehicle comprising: an engine; a transmission; a clutch configured to couple the engine and transmission during engine starts; and a controller programmed to, in response to an actual engine start time being less than a lower threshold time for an engine start event, alter an engine start torque apply schedule for the clutch during a next engine start event such that the actual engine start time is greater than the lower threshold time.
 2. The vehicle of claim 1, wherein the controller is further programmed to, in response to the actual engine start time being greater than an upper threshold time for the engine start event, alter the engine start torque apply schedule for the clutch during the next engine start event such that the actual engine start time is less than the upper threshold time.
 3. The vehicle of claim 2, wherein the actual engine start time is defined by a time period beginning with initial contact of the clutch and ending with the engine obtaining a first firing top dead center position, and the upper and lower threshold times define a target time range for the time period.
 4. The vehicle of claim 3, wherein the controller is further programmed to alter the engine start torque apply schedule before the next engine start event.
 5. The vehicle of claim 4, wherein the engine start torque apply schedule is altered based on a ratio of the actual engine start time to a time falling within the target time range.
 6. The vehicle of claim 4, wherein the engine start torque apply schedule is altered based on a difference between the actual engine start time and a target time that is within the target time range.
 7. The vehicle of claim 4, wherein the actual engine start time is an average of actual engine start times of a plurality of previously executed engine starts.
 8. The vehicle of claim 2, wherein the engine start torque apply schedule is altered based on an engine coolant temperature, an engine oil temperature, or a barometric pressure.
 9. The vehicle of claim 2, wherein the upper and lower threshold times have a same value.
 10. The vehicle of claim 2, wherein the actual engine start time is defined by a time period beginning with the engine obtaining a first firing top dead center position and ending with a speed of the engine becoming equal to an output speed of the clutch, and wherein the upper and lower threshold times define a target time range for the time period.
 11. The vehicle of claim 10, wherein the controller is further programmed to alter the engine start torque apply schedule before the next engine start event.
 12. The vehicle of claim 11, wherein the engine start torque apply schedule is altered based on a ratio of the actual engine start time to a time falling within the target time range.
 13. The vehicle of claim 11, wherein the engine start torque apply schedule is altered based on a difference between the actual engine start time and a time falling within the target time range.
 14. A method of operating a clutch configured to couple an engine to a transmission of a vehicle during engine starts comprising: in response to an actual engine start time being less than a lower threshold time for an engine start event, altering an engine start torque apply schedule for the clutch during a subsequent engine start event such that the actual engine start time is greater than the lower threshold time.
 15. The method of claim 14, further comprising: in response to the actual engine start time being greater than an upper threshold time for the engine start event, altering the engine start torque apply schedule during the subsequent engine start event such that the actual engine start time is less than the upper threshold time.
 16. The method of claim 15, wherein the upper and lower threshold times define a target time range, and the engine start torque apply schedule is altered based on a ratio of the actual engine start time to a time falling within the target time range.
 17. The method of claim 15, wherein the upper and lower threshold times define a target time range, and the engine start torque apply schedule is altered based on a difference between the actual engine start time and a time falling within the target time range.
 18. A vehicle comprising: an engine; a transmission; a clutch configured to couple the engine to the transmission during engine starts; and a controller programmed to, in response to an actual engine start time falling outside a target time range, alter an engine start torque apply schedule for the clutch during a subsequent engine start event such that the actual engine start time falls within the target time range.
 19. The vehicle of claim 18, wherein altering the engine start torque apply schedule includes altering a hydraulic pressure apply schedule for the clutch. 